The subject invention relates generally to positioning systems and more particularly to improvements in a servo-control continuous path closed-loop position control of an object.
In the art of precision positioning systems to position a machine tool slide, for example, it has generally been the practice to utilize a rotary prime mover coupled to a ball nut/lead screw. Linear slide motion is obtained by attaching the ball nut to the machine slide while driving the lead screw through a gear box. The prime mover may take various rotary drive forms, such as servo motors, pulse motors or pneumatic drives. A numerical controller may be employed to generate position command signals for multiple axis coordinated continuous position control of a machine tool from part description data stored on punched tape or other storage media. Continuous-path-positioning as opposed to point-to-point positioning requires extremely precise control of the velocity on the object being positioned at all times.
Various advances in the art have been used to improve the tool-positioning accuracy in the above-described machining system. For example, the laser interferometer has been employed to monitor the slide movement and its output compared with the command signals to obtain a following error signal which is used to power the prime mover through a servo amplifier. A velocity feedback arrangement has been combined with the laser interferometer position feedback to improve position accuracy. Normally a rotary transducer, such as a tachometer generator, is attached to the lead screw to generate the velocity control feedback signal. Nevertheless, prior art continuous-path-position control systems were limited with respect to their prospective utility in many applications, especially diamond machining of mirror surface finishes of complex geometric configurations. For continuous-path-precision machining systems, the above-described systems cannot be controlled to obtain continuous-path following errors in the range required for precise surface finishes.
For a specific feed rate, the error signal will have two significant components: a steady state following error level and a transient error level riding the following error level. The small steady state error is required to reduce the effects of load disturbances while a small transient error is required for a good surface texture.
Large loop gains are generally necessary for the position and velocity loops in order to reduce the error signal. A conventional way for achieving this is to use a lag compensator in the forward path. In some cases, this signal compensator is not adequate, particularly in ultra-precision machining. A second lag compensator added to the outer loop may be used to further reduce the error signal. This arrangement, either the single inner loop lag compensator or the combination of inner and outer loop lag compensation, is effective for routine cutting speeds (i.e., 0.05 inch per minute and higher).
At low cutting speeds, however, even very low friction in the drive train will cause a sticking and then slipping effect. It is believed that the slow transient response of the lag compensator allows the error signal to reach several microinches before a large enough portion of the signal has reached the terminals of the motor to effect the change in position, thus causing the stick-slip movement of a tool holder on the axis slide, or the like. The result is a finish on the machined part that is inferior to that obtained at higher cutting speeds. However, slow cutting speeds cannot be avoided when preparing certain parts.
Therefore, there is a need for improvements in continuous-path-position control systems for precision, continuous-path contour machining which can be operated at very low velocities and yet obtain an acceptable surface finish by maintaining transient positioning errors for the individual machine slides to less than 3 microinches.